1. Field of the Invention
The present invention relates to a method for fabricating a mask read only memory (ROM), and more particularly to a method for fabricating a mask ROM of the ion implantation type.
2. Description of the Prior Art
ROMs are memory devices capable of storing information, without any change, for the operating life thereof. They are a kind of non-volatile memory device in which information stored is not erased even if the supply of electric power were to be shut off.
Such ROMs are widely employed in look-up tables and character generators.
ROMs are mainly classified into mask ROMs (MROMs), in which data desired by the user is written by use of a mask in a fabricating process for the devices, and programmable ROMs (PROMs), in which data is electrically programmed after the fabrication. PROMs are further classified into erasable PROMs (EPROMs), in which data stored is erasable by use of ultraviolet ray so that new data can then be programmed, and electrically erasable ROMs (EEPROMs), in which data stored is electrically erasable so that new data can then be programmed.
MROMs in which data desired by the user is written into the ROM by use of a mask in a fabricating process for the devices have several advantages. A first is the advantage of having a simple circuit construction in which no separate data writing circuit is required. A second is the advantage of requiring only a simple fabrication in which the memory cell structure used requires no specific fabricating process. A third advantage lies in providing various memory cell systems from which an appropriate one can be selected according to the use intended.
However, such MROMs have a disadvantage because the MROMs are fabricated according to an order for codes, the time (turn around time) taken in supplying products from the manufacturer to the final user is lengthy, as compared with other ROMs such as EPROMs and EEPROMs. The MROMs also have the disadvantage of being difficult to produce having multiple codes in small quantities. Moreover, there is the drawback that the manufacturer or the user may be taking more or less of a risk, since it is impossible to change data after the fabrication because the programming of data is achieved during the fabrication.
Data writing methods for MROMs widely used are those of a contact opening and closing method, wherein writing of information is achieved by opening and closing contacts, and an ion implantation method, wherein writing of information is achieved by use of ion implantation. MROMs are mainly classified into NOR type MROMs and NAND type MROMs as defined by the circuit structure of the memory cell which is used in the device.
FIGS. 1A and 1B are pictorial schematic views respectively illustrating a memory cell array in a conventional MROM of the contact opening and closing type. FIG. 1A is a plan view of a layout of the memory cell array, whereas FIG. 1B is a schematic circuit diagram of an equivalent circuit of the memory cell array.
In FIG. 1A, a region 1 surrounded by the dotted line corresponds to one of a plurality of memory cells arranged in a matrix manner in the memory cell array. Each memory cell 1 is realized by either an N type or a P type MOS transistor.
Since writing of data in the MROM of the contact opening and closing type is achieved according to either the presence of a contact hole 5 or the absence thereof, the opening and closing of contact hole 5, namely, the presence or absence of contact holes 5, corresponds to logic levels "1" and "0" of the stored data, respectively.
In the MROM of the contact opening and closing type, a diffusion region 2 which serves as a drain region of the MOS transistor, is connected to a bit line 6 through each corresponding contact hole 5. Thus the process of forming ROM codes becomes nearer to a final step in the fabrication process. As a result, the processing time required to be taken in fabricating the MROM by the manufacturer after the ordering by the consumer, i.e., the "turn around time" (TAT), is reduced.
FIGS. 2A and 2B are schematic and pictorial views respectively illustrating a memory cell array in a conventional MROM of the ion implantation type. FIG. 1A is a plan view of a layout of such a memory cell array, whereas FIG. 1B is a circuit diagram of an equivalent circuit of the memory cell array.
In FIGS. 2A and 2B, elements corresponding to those in FIGS. 1A and 1B are denoted by the same reference numerals.
The MROM shown in FIG. 2A is of the ion implantation type, in which a region 7 surrounded by the dotted line corresponds to an impurity-implanted region formed at a channel region of a MOS transistor.
In this case, the memory cell 1 comprises a plurality of MOS transistors which include those each having impurity-implanted regions 7 and others not having an impurity-implanted region 7. The MOS transistors having the impurity-implanted region 7 become transistors operating in the depletion mode corresponding to the level "1" of information, whereas the MOS transistors having no impurity-implanted region 7 become transistors operating in the enhancement mode corresponding to a logic level "0" of information.
In FIG. 2B, the reference numerals E and D denote MOS transistors of the enhancement and the depletion modes, respectively.
In an MROM of the ion implantation type, the ion implantation is carried out at a point of time nearer to the initial process step than to the final process step. Thus the processing time for fabricating an MROM by the manufacturer after an order by a user, the TAT, is lengthened, as compared to MROMs of the contact opening and closing type.
For fabricating such conventional ion implantation MROMs, first, a gate oxide film, a gate electrode and source/drain regions are formed on a semiconductor substrate, forming a MOS transistor. Thereafter, an ion implantation step for introducing ROM codes is carried out, so that data in accordance with the user's order is programmed.
A boron phosphorous silicate glass (BPSG) film is then deposited over the MOS transistor. This BPSG film is then heat treated to form a smoothing film. Thereafter, the steps for formation of contact holes and metal film are carried out. A passivation layer is then deposited over the entire exposed surface of the resulting structure. Thereafter, formation of contact pads is carried out, to complete the fabrication of the MROM.
In such a conventional method, there is a problem of a low productivity, because the process of implanting ROM code ions is carried out at an initial stage of the fabrication, which results in a very long turn around time, in that the fabrication of MROM is achieved after an order for ROM codes is placed by the user, at which time ROM code ions are implanted by the manufacturer in accordance with the user's order.